Integration, multi-functionalization and speed increase have recently been advancing in LSIs in various electronic apparatuses, requiring their power supplies to have higher power output. Taking note PCs for example, DC-DC converters have been required to provide larger current due to multi-functionalization and quality enhancement prompted by the speed-up of CPUs, the capacity and speed increase of memory devices, etc. Also, higher integration of parts causes electronic parts to generate more heat, elevating their environment temperature to near 100° C. Accordingly, DC-DC converters contained in note PCs comprising high-performance CPUs are required to be able to supply large current at actual environment temperatures.
DC-DC converters, etc. used in electric vehicles, hybrid vehicles, etc. are also operated in wide temperature ranges, so that they are required to exhibit enough performance even at 100° C. or higher. Accordingly, these in-vehicle DC-DC converters, etc. are required to be adaptable to higher temperatures and larger current.
Adaptability to higher temperatures and larger current is also required by choke coils constituting DC-DC converters, and their parts, magnetic cores. The choke coils are required to have high inductance even when large current is supplied at high temperatures. The magnetic cores are required to be usable at a frequency of several hundreds of kHz, and resistant to magnetic saturation even when large current is supplied at high temperatures.
Magnetic cores for choke coils, etc. are made of soft-magnetic metals such as silicon steel, amorphous alloys, soft-magnetic, fine-crystal alloys, etc., or ferrites. Although the soft-magnetic metals have higher saturation magnetic flux densities than those of ferrites, thus resistant to magnetic saturation even when large current is supplied, they are disadvantageous in a high cost, and low resistance that makes use at high frequencies impossible. On the other hand, the soft-magnetic ferrites can advantageously be used at high frequencies because of higher resistance than the soft-magnetic metals, in addition to a low cost. Among the soft-magnetic ferrites, Mn—Zn ferrite is suitable for large-current cores, because it has a higher saturation magnetic flux density than that of Ni—Zn ferrite.
Including those used for choke coils for DC-DC converters, conventional Mn—Zn ferrite generally comprises about 50-55% by mol of Fe2O3, and it is known that increase in the Fe2O3 content leads to a higher maximum magnetic flux density. However, when as much Fe2O3 as more than 60% by mol is contained, it has been difficult to produce sintered Mn—Zn ferrite having a high maximum magnetic flux density by a powder metallurgy method for the reasons described below, though single-crystal Mn—Zn ferrite has a high maximum magnetic flux density. In the sintering step of Mn—Zn ferrite, oxygen should be released from Fe2O3 in the spinelization reaction of reducing Fe2O3 to FeO, but the release of oxygen is insufficient in a composition with much excess Fe2O3, resulting in the likelihood that Fe2O3 remains as an undesirable phase (hematite phase), thus failing to obtain high magnetic properties (high magnetic flux density). In addition, because the spinelization reaction and the sintering are hindered, it is impossible to obtain a high-density sintered body, inevitably failing to a high maximum magnetic flux density.
The magnetic properties of ferrite generally tend to be influenced by temperatures. Particularly Mn—Zn ferrite has a high maximum magnetic flux density at room temperature, but its maximum magnetic flux density decreases as the temperature is elevated. The maximum magnetic flux density at a high temperature of about 100° C. is usually as low as about 75-80% of that at room temperature. Such reduction of a maximum magnetic flux density leads to the deterioration of DC bias current characteristics when used in choke coils. To obtain a high maximum magnetic flux density at a high temperature of about 100° C., it is necessary to compensate the reduction of a maximum magnetic flux density with temperature, by increasing the maximum magnetic flux density at room temperature, or by decreasing the reduction ratio of a maximum magnetic flux density as the temperature is elevated.
JP6-333726A discloses a method for producing Mn—Zn ferrite having a high maximum magnetic flux density without undesirable phases such as a wustite phase, a hematite phase, etc., by sintering a ferrite material comprising 62-68% of Fe2O3, 16-28% of MnO and 10-16% of ZnO by mol as main components, and at least one of CaO, SiO2, ZrO2 and CoO as a sub-component, together with an organic binder as a reducing agent in a inert gas. However, the composition described in JP6-333726A cannot provide the resultant sintered body with a sufficient maximum magnetic flux density at room temperature, and the maximum magnetic flux density decreases largely as the temperature is elevated. Accordingly, it is difficult to produce Mn—Zn ferrite having a high maximum magnetic flux density at a high temperature of 100° C.
JP11-329822A discloses a sintered Mn—Zn ferrite body having a high maximum magnetic flux density particularly at a high temperature of 100° C., which comprises 60-85% by mol of iron oxide, and 0-20% by mol of zinc oxide, the balance being manganese oxide, and has as high a maximum magnetic flux density as 450 mT or more at 100° C., with a small reduction ratio of a maximum magnetic flux density with temperature. However, despite the excess-Fe composition (as high Fe2O3 as more than 60% by mol), which is inherently expected to provide a high maximum magnetic flux density, sintered Mn—Zn ferrite has a density of less than 4.9 g/cm3, not on a sufficient level as compared with the theoretical density of 5.1-5.2 g/cm3. Further, the above excess-Fe composition may generate undesirable phases such as a hematite phase, etc. depending on the variations of production conditions, making it difficult to stably obtain Mn—Zn ferrite having a high maximum magnetic flux density.
As described above, the above maximum magnetic flux densities of the conventional Mn—Zn ferrites do not satisfy the requirements of increasingly higher temperatures and larger current. Thus, ferrites having higher maximum magnetic flux densities and choke coils adaptable to larger current are desired.
In the case of producing a sintered ferrite body having a composition with much excess Fe, a spinelization reaction should be accelerated and controlled more than usual Mn—Zn ferrites comprising 50-55% by mol of Fe2O3. Also, because undesirable phases such as a hematite phase, etc. are easily formed in the spinelization reaction, it is difficult to achieve a high maximum magnetic flux density with good reproducibility. When an organic binder is added as a reducing agent, too, its amount is limited from the aspect of moldability, and because its effects are different depending on the ferrite compositions, etc., it is difficult to obtain a sintered ferrite body having a high maximum magnetic flux density with good reproducibility.